Statement. Fog is a cloud whose base touches the ground; weather services call it fog when visibility drops below one kilometre. On the road to Coentral das Barreiras (about 725 m, Serra da Lousã), we drove straight into one, and the kids reached out of the windows to touch it.
On the way up to Coentral das Barreiras, the fog closed in so completely that the kids announced we were inside a cloud. Then they put their hands out of the car windows to touch it. This post checks whether they were right. Spoiler: they were, and physics has the paperwork.
Is fog a cloud?
Yes. Fog and clouds are the same thing: tiny water droplets suspended in the air. The only difference is where they sit. When a cloud reaches the ground, meteorologists call it fog, and the formal threshold is a horizontal visibility of less than one kilometre. Coentral das Barreiras sits at about 725 metres on the southern slope of the Serra da Lousã, so when the clouds came down the serra, we simply drove into one.
What did the hands find? Water, mostly. A cloud feels like nothing and leaves you damp, which is also a fair description of some meetings.
Why is fog white if the sky is blue?
Air molecules are far smaller than the wavelength of visible light, and they scatter short (blue) wavelengths much more than long (red) ones: Rayleigh scattering grows as 1/λ⁴. That is why the sky is blue. Fog droplets are much larger, comparable to or bigger than the wavelength, and in that regime (Mie scattering) all colours scatter roughly equally. Equal parts of every colour add up to white. The cloud has no favourites.
Why does everything disappear so fast in fog?
Because contrast decays exponentially with distance. The apparent contrast of an object against its background follows C = C₀·e^(−βd), where β is the extinction coefficient of the air. Koschmieder turned this into a rule of thumb for visibility: V ≈ 3.9/β, the distance at which contrast falls to 2%, roughly the threshold of the human eye. In thick fog β is large, so the world ends a few metres beyond the bonnet.
That is also why, in the photo above, each tree looks like a paler copy of the one in front. It is not a mood. It is a curve.
Theorem: the kids were right
Putting the lemmas together: we were inside a cloud (fog is a cloud on the ground), it looked white because its droplets treat every colour the same, and the road disappeared exponentially. The hands out of the window were an experiment with a sample size of two and a very clear result: clouds are wet. More about the place in the Coentral das Barreiras proof.
Driving in mountain fog: practical corollaries
- Dipped headlights, not full beam. Fog scatters light back towards you; more light only means a brighter wall. Use fog lights if your car has them.
- Slow down. Exponential decay is not kind to braking distances.
- Let the kids do the science. Hands out of the window only at crawling speed, and on the side away from traffic.
- Don’t panic. Douglas Adams printed it on the cover of the Guide for a reason. Mountain fog in the Serra da Lousã often lifts as fast as it arrives.
Next time someone tells you their head is in the clouds, ask them for the extinction coefficient.
Frequently asked questions
Is fog a cloud?
Yes. Fog is a cloud whose base touches the ground. Meteorologists classify it as fog when horizontal visibility drops below one kilometre.
Why is fog white?
Fog droplets are large compared with the wavelength of light, so they scatter all colours roughly equally (Mie scattering). Equal amounts of every colour look white.
Why is the sky blue but fog white?
Air molecules are tiny and scatter blue light much more than red (Rayleigh scattering, proportional to 1/λ⁴). Fog droplets are big and scatter every colour almost equally.
Should I use high beams in fog?
No. Use dipped headlights and fog lights. High beams reflect off the droplets and reduce visibility further.